Chemically inert megasonic transducer system

ABSTRACT

A megasonic cleaning system comprised of a container, a resonator, a piezoelectric crystal and an indium layer for attaching the resonator to the piezoelectric crystal. The container includes a fluid chamber for holding a volume of cleaning solution. The resonator is selected from the group consisting of sapphire, quartz, silicon carbide, silicon nitride and ceramic. The resonator forms the bottom of the container and has an interface surface which abuts the fluid chamber.

This application is a continuation-in-part of Ser. No. 09/384,947, filedAug. 27, 1999. This application also claims priority of U.S. provisionalpatent application No. 60/154,481, filed Sep. 17, 1999.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to megasonic cleaning systems havingchemically inert resonators bonded to the piezoelectric crystal and moreparticularly to systems in which indium is used to bond the resonator tothe crystal.

2. Background Information

It is well-known that sound waves in the frequency range of 0.4 to 2.0megahertz (MHZ) can be transmitted into liquids and used to cleanparticulate matter from damage sensitive substrates. Since thisfrequency range is predominantly near the megahertz range, the cleaningprocess is commonly referred to as megasonic cleaning. Among the itemsthat can be cleaned with this process are semiconductor wafers invarious stages of the semiconductor device manufacturing process, diskdrive media, flat panel displays and other sensitive substrates.

Megasonic acoustic energy is generally created by exciting a crystalwith radio frequency AC voltage. The acoustical energy generated by thecrystal is passed through an energy transmitting member and into thecleaning fluid. Frequently, the energy transmitting member is a wall ofthe vessel that holds the cleaning fluid. The crystal and its relatedcomponents are referred to as a megasonic transducer. For example, U.S.Pat. No. 5,355,048, discloses a megasonic transducer comprised of apiezoelectric crystal attached to a quartz window by several attachmentlayers. The megasonic transducer operates at approximately 850 KHz.Similarly, U.S. Pat. No. 4,804,007 discloses a megasonic transducer inwhich energy transmitting members comprised of quartz, sapphire, boronnitride, stainless steel or tantalum are glued to a piezoelectriccrystal using epoxy.

It is also known that piezoelectric crystals can be bonded to certainmaterials using indium. For example, U.S. Pat. No. 3,590,467 discloses amethod for bonding a piezoelectric crystal to a delay medium usingindium where the delay medium comprises materials such as glasses, fusedsilica and glass ceramic.

A problem with megasonic transducers of the prior art is that theacoustic power that can be generated by the megasonic transducer in thecleaning solution is limited to about 10 watts per cm² of activepiezoelectric surface without supplying additional cooling to thetransducer. For this reason, most megasonic power sources have theiroutput limited, require liquid or forced air cooling or are designed fora fixed output to the piezoelectric transducer or transducers.Typically, fixed output systems are limited to powers of 7-8 watts/cm².This limits the amount of energy that can be transmitted to the cleaningsolution. If more power is applied to the transducer, the crystal canheat up to the point where it becomes less effective at transmittingenergy into the cleaning solution. This is caused either by nearing themaximum operating temperature of the crystal or, more often, by reachingthe failure temperature of the material used to attach the crystal tothe energy transmitting means.

Another problem with prior art cleaning systems that utilize megasonictransducers, is that there is no practical way of replacing a defectivetransducer once the transducer has been attached to the cleaning system.This means that users have to incur large expenses to replace defectivetransducers, for example by purchasing a whole new cleaning vessel.

SUMMARY OF THE PRESENT INVENTION

Briefly, the present invention is a megasonic cleaning system comprisedof a container, a resonator, a piezoelectric crystal and an indium layerfor attaching the resonator to the piezoelectric crystal. The containerincludes a fluid chamber for holding a volume of cleaning solution. Theresonator is selected from the group consisting of sapphire, quartz,silicon carbide, silicon nitride and ceramic. The resonator forms thebottom of the container and has an interface surface which abuts thefluid chamber. This orientation means that the interface surface is incontact with at least some of the cleaning solution when cleaningsolution is present in the fluid chamber.

The piezoelectric crystal is capable of generating acoustic energy inthe frequency range of 0.4 to 2.0 MHz when power is applied to thecrystal. The attachment layer is comprised of indium and is positionedbetween the resonator and the piezoelectric crystal so as to attach thepiezoelectric crystal to the energy transmitting member. A firstadhesion layer comprised of chromium, copper and nickel is positioned incontact with a surface of the piezoelectric crystal. A first wettinglayer comprised of silver is positioned between the first adhesion layerand the bonding layer for helping the bonding layer bond to the firstadhesion layer. A second adhesion layer comprised of chromium, copperand nickel is positioned in contact with a surface of the resonator. Asecond wetting layer comprised of silver is positioned between thesecond adhesion layer and the bonding layer for helping the bondinglayer bond to the second adhesion layer.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an acoustic transducer assemblyaccording to the present invention;

FIG. 2 is a side view of an acoustic transducer according to the presentinvention;

FIG. 3 is side view of a spring/button electrical connector boardaccording to the present invention;

FIG. 4 is an exploded view of the acoustic transducer assembly accordingto the present invention;

FIG. 5 is a side view of an acoustic transducer according to the presentinvention;

FIG. 6 is an exploded view of a megasonic cleaning system according tothe present invention; and

FIG. 7 is a schematic circuit diagram of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a cross section of an acoustic transducer assembly 10comprised of an acoustic transducer 14, a spring/button electricalconnector board 18 and a housing 22. The transducer 14 comprises aresonator 26 which is bonded to a piezo crystal 30. The electricalconnector board 18 comprises a printed circuit board (PCB) 34 which hasa plurality of first spring/button connectors 38 and a plurality ofsecond spring/button connectors 42 connected to it. The housing 22 is acase that encloses the electrical connector board 18 so that it isprotected from the environment. The electrical connector board 18 andthe acoustic transducer 14 sit in a cavity 46 inside the housing 22.

The resonator 26 forms part of a wall in the housing 22 that covers andseals the cavity 46. A surface 50 of the resonator 26 forms an externalside of the acoustic transducer assembly 10. In the preferredembodiment, the acoustic transducer 14 is used to generate megasonicacoustic energy in a cleaning apparatus used to clean semiconductorwafers. The surface 50 will be in contact with the cleaning fluid usedin the cleaning apparatus.

FIG. 2 illustrates that the acoustic transducer 14 comprises thepiezoelectric crystal 30 attached to resonator 26 by an indium layer 60.In the preferred embodiment, a plurality of other layers are disposedbetween the piezoelectric crystal 30 and the resonator 26 to facilitatethe attachment process. Specifically, a first metal layer 64 is presentadjacent to a front surface 68 of the indium layer 60. A second metallayer 72 is present adjacent to a back surface 76 of the indium layer60. A blocking layer 80 is positioned between the metal layer 72 and thepiezoelectric crystal 30 to promote adhesion. In the preferredembodiment, the blocking layer 80 comprises a chromium-nickel alloy, andthe metal layers 64 and 72 comprise silver. The blocking layer 80 has aminimum thickness of approximately 500 Å and the metal layer 72 has athickness of approximately 500 Å.

In the preferred embodiment, the piezoelectric crystal 30 is comprisedof lead zirconate titanate (PZT). However, the piezoelectric crystal 30can be comprised of many other piezoelectric materials such as bariumtitanate, quartz or polyvinylidene fluoride resin (PVDF), as iswell-known in the art. In the preferred embodiment, two rectangularlyshaped PZT crystals are used in the transducer 14, and each PZT crystalis individually excited.

A blocking/adhesion layer 84 separates the metal layer 64 from theresonator 26. In the preferred embodiment, the blocking/adhesion layer84 comprises a layer of nickel chromium alloy which is approximately 500Å thick. However, other materials and/or thicknesses could also be usedas the blocking layer 84. The function of the blocking layer 84 is toprovide an adhesion layer for the metal layer 64. In the preferredembodiment, the metal layer 64 comprises silver and has a thickness ofapproximately 500 Å. However, other metals and/or thicknesses could beused for the metal layer 64. The function of the metal layer 64 is toprovide a wetting surface for the molten indium.

An additional layer is also disposed on a back side of the piezoelectriccrystal 30. Specifically, a metal layer 86 is positioned on the backside of the piezoelectric crystal 30 and covers substantially all of thesurface area of the back side of the crystal 30. Generally, the layer 86is applied to the piezoelectric crystal 30 by the manufacturer of thecrystal. The layer 86 functions to conduct electricity from a set of thespring/button connectors shown in FIG. 1, so as to set up a voltageacross the crystal 30. Preferably, he metal layer 86 comprises silver,nickel or another electrically conductive layer.

In the preferred embodiment, the indium layer 60 comprises pure indium(99.99%) such as is commercially available from Arconium or Indalloy.However, Indium alloys containing varying amounts of impurity metals canalso be used, albeit with less satisfactory results. The benefit ofusing pure indium and its alloys is that indium possesses excellentshear properties that allow dissimilar materials with differentcoefficients of expansion to be attached together and experience thermalcycling without damage to the attached materials.

In the preferred embodiment, the resonator 26 is a piece of sapphire(Al₂O₃). Preferably, the sapphire is high grade having a designation of99.999% (5,9s+purity). However, other materials, such as stainlesssteel, tantalum, aluminum, silica compounds, such as quartz, ceramicsand plastics, can also finction as the resonator 26. The purpose of theresonator 26 is to separate (isolate) the piezoelectric crystal 30 fromthe fluid used in the cleaning process, so that the fluid does notdamage the crystal 30. Thus, the material used as the resonator 26 isusually dictated, at least in part, by the nature of the fluid. Theresonator 26 must also be able to transmit the acoustic energy generatedby the crystal 30 into the fluid. Sapphire is a desirable material forthe resonator 26 when the items to be cleaned by the megasonic cleaningapparatus require parts per trillion purity. For example, semiconductorwafers require this type of purity.

In the preferred embodiment, the resonator 26 has a thickness “e” whichis preferably a multiple of one-half of the wavelength of the acousticenergy emitted by the piezoelectric crystal 30, so as to minimizereflectance problems. For example, “e” is approximately six millimetersfor sapphire and acoustic energy of about 925 KHz.

FIG. 3 illustrates the spring/button electrical connector board 18 inmore detail. Each first spring/button connector 38 comprises an uppersilver button 90 and a lower silver button 94. The upper silver button90 and the lower silver button 94 are attached to a plated silver spring98 and soldered to the printed circuit board (PCB) 34 so that theconnector 38 can provide an electrical connection to the acoustictransducer 14. The upper silver button 90 has a thickness “t” of about0.15 inches.

Similarly, each second spring/button connector 42 comprises an uppersilver button 98 and a lower silver button 102. The upper silver button98 and the lower silver button 102 are attached to a silver platedspring 106 and soldered to the PCB 34 so that the connector 42 canprovide an electrical connection to the acoustic transducer 14. Theupper silver button 98 has a thickness “r” of about 0.10 inches.Generally, the thickness “t” is greater than the thickness “r” becausethe first spring/button connector 38 has extend farther up to makecontact with the acoustic transducer 14 than does the secondspring/button connector 42 (see FIGS. 1 and 2).

A radio frequency (RF) generator provides a voltage to the PCB 34. ThePCB 34 includes electrical connections to the spring/button connectors38 and 42 so that the polarity of the spring/button connectors 38 ispositive and the polarity of the spring/button connectors 42 isnegative, or vice versa. Examination of FIG. 2 shows that in theacoustic transducer 14, the layers 26, 84 and 64 have a greater length“j” than the length “k” of the layers 60, 72, 80, 30 and 86. Thiscreates a step-region 110 on the silver layer 64 that can be contactedby the upper buttons 90 of the spring/button connectors 38. The upperbuttons 98 of the spring/button connectors 42 make electrical contactwith the silver layer 86.

The purpose of the spring/button connectors 38 and 42 is to create avoltage difference across the piezoelectric crystal 30 so as to exciteit at the frequency of the RF voltage supplied by the RF generator. Theconnectors 38 connect the metal layer 64 to the RF generator. Theconnectors 42 connect the layer 86 to the RF generator. The RF generatordelivers a RF alternating current to the piezoelectric crystal 30 viathe connectors 38 and 42. Preferably, this is a 925 KHz signal, at 600watts of power. The effective power in the piezoelectric crystal 30 isapproximately 15.5 watts/cm². The effective power in the piezoelectriccrystal 30 is defined as the forward power into the crystal 24 minus thereflected power back into the RF generator. Thus, the step-region 110,and the spring/button connectors 38 and 42, allow a voltage to be set upacross the piezoelectric crystal 30 without the need for solderingdiscrete leads to the layers 64 and 86.

In FIG. 3, a plurality of electrical components 114, such as capacitorsand/or inductors, are shown. These are used to balance the impedancebetween the RF input and the spring output.

FIG. 4 illustrates the way the acoustic transducer 14, the spring/buttonelectrical connector board 18 and the housing 22 fit together to formthe acoustic transducer assembly 10.

The acoustic transducer 14 is prepared as follows (using the preferredmaterials described previously): Assuming that the resonator 26 issapphire, the surface of the sapphire that will be adjacent to the layer84 is cleaned by abrasive blasting or chemical or sputter etching. Theblocking/adhesion layer 84 is then deposited on the resonator 26 byphysical vapor deposition (“PVD”), such as argon sputtering. A platingtechnique could also be used. The silver layer 64 is then deposited onthe chromium blocking/adhesive layer 84 using argon sputtering. Aplating technique could also be used.

The piezoelectric crystal 30 is usually purchased with the layers 86already applied to it. The blocking layer 80 and the metal layer 72 aredeposited on the crystal 30 by plating or physical vapor deposition.

The resonator 26 and the piezoelectric crystal 30 are both heated toapproximately 200° C., preferably by placing the resonator 26 and thecrystal 30 on a heated surface such as a hot-plate. When both pieceshave reached a temperature of greater than 160° C., solid indium isrubbed on the surfaces of the resonator 26 and the crystal 30 which areto be attached. Since pure indium melts at approximately 157° C., thesolid indium liquefies when it is applied to the hot surfaces, therebywetting the surfaces with indium. It is sometimes advantageous to addmore indium at this time by using the surface tension of the indium toform a “puddle” of molten indium.

The resonator 26 and the piezoelectric crystal 30 are then pressedtogether so that the surfaces coated with indium are in contact witheach other, thereby forming the transducer 14. The newly formedtransducer 14 is allowed to cool to room temperature so that the indiumsolidifies. Preferably, the solid indium layer has a thickness “g” whichis just sufficient to form a void free bond (i.e. the thinner thebetter). In the preferred embodiment, “g” is approximately one mil(0.001 inches). Thicknesses up to about 0.01 inches could be used, butthe efficiency of acoustic transmission drops off when the thickness “g”is increased.

Preferably, the transducer 14 is allowed to cool with the piezoelectriccrystal 30 on top of the resonator 26 and the force of gravity holdingthe two pieces together. Alternatively, a weight can be placed on top ofthe piezoelectric crystal 30 to aide in the bonding of the indium.Another alternative is to place the newly formed transducer 14 in aclamping fixture.

Once the transducer 14 has cooled to room temperature, any excess indiumthat has seeped out from between the piezoelectric crystal 30 and theresonator 26, is removed with a tool or other means.

FIG. 5 illustrates a preferred embodiment of an acoustic transducersystem 124 in which the resonator can be one of several chemically inertmaterials. These materials include sapphire, quartz, silicon carbide,silicon nitride and ceramics. The transducer system 124 shown in FIG. 5is similar to the transducer 14 shown in FIG. 2. However, several of theattachment layers used in the transducer system 124 are different.

In FIG. 5, the acoustic transducer system 124 comprises a piezoelectriccrystal 130 attached to a resonator 134 by a bonding layer 138. Aplurality of attachment layers are disposed between the piezoelectriccrystal 130 and the resonator 134 to facilitate the attachment process.Specifically, a second wetting layer 142 is present adjacent to a frontsurface 146 of the bonding layer 138. A first wetting layer 150 ispresent adjacent to a back surface 154 of the bonding layer 138. A firstadhesion layer 158 is positioned between the first wetting layer 150 andthe piezoelectric crystal 130 to facilitate the mechanical adhesion ofthe bonding layer 138 to the crystal 130.

In the preferred embodiment, the first adhesion layer 158 comprises anapproximately 5000 Å thick layer of an alloy comprised of chrome and anickel copper alloy, such as the alloys marketed under the trademarksNickel 400™ or MONEL™. However, other materials and/or thicknesses couldalso be used as the first adhesion layer 158. Nickel 400™ and MONEL™ arecopper nickel alloys comprised of 32% copper and 68% nickel.

Preferably, the wetting layers 142 and 150 comprise silver. The wettinglayers 142 and 150 each have a thickness of approximately 5000 Å.However, other metals and/or thicknesses could be used for the wettinglayers 142 and 150. The function of the wetting layers 142 and 150 is toprovide a wetting surface for the molten indium, meaning that the layers142 and 150 help the bonding (indium) layer 138 adhere to the firstadhesion layer 158 and a second adhesion layer 162, respectively. It isthought that the silver in the wetting layers 142 and 150 forms an alloywith the indium, thereby helping the bonding layer 138 adhere to theadhesion layers 158 and 162. The transducer system 124 includes astep-region 195 in the wetting layer 142 which is exactly analogous tothe step-region 110 described previously with respect to FIG. 2.

In the preferred embodiment, the piezoelectric crystal 130 is identicalto the piezoelectric crystal 30 already described, and is comprised oflead zirconate titanate (PZT). However, many other piezoelectricmaterials such as barium titanate, quartz or polyvinylidene fluorideresin (PVDF), may be used as is well-known in the art. In the preferredembodiment, four rectangularly shaped PZT crystals are used in thetransducer 14 (shown in FIG. 6), and each PZT crystal is individuallyexcited. However, other numbers of the crystals 130 can be used,including between one and sixteen of the crystals 130, and other shapes,such as round crystals, could be used.

The second adhesion layer 162 separates the second wetting layer 142from the resonator 134. In the preferred embodiment, the adhesion layer162 comprises an approximately 5000 Å thick layer of an alloy comprisedof chrome and a nickel copper alloy, such as the alloys marketed underthe trademarks Nickel 400™ or MONEL™. However, other materials and/orthicknesses could also be used as the second adhesion layer 162.

The function of the first adhesion layer 158 is to form a strong bondbetween the bonding (indium) layer 138 and the piezoelectric crystal130. As noted previously, the wetting layer 150 forms an alloy with theindium in the bonding layer 138, thereby permitting the adhesion layer158 to bond with the bonding layer 138. Similarly, the function of thesecond adhesion layer 162 is to form a strong bond between the bonding(indium) layer 138 and the resonator 134. The wetting layer 142 forms analloy with the indium in the bonding layer 138, thereby permitting theadhesion layer 162 to bond with the bonding layer 138. Additionally, thefirst adhesion layer 158 needs to be electrically conductive in order tocomplete the electrical path from the step region 195 to the surface ofthe piezoelectric crystal 130. Furthermore, the adhesion layers 158 and162 may prevent (block) the indium in the bonding layer 138 fromreacting with the crystal 130 and/or the resonator 134, respectively.

An additional two layers are disposed on a back side of thepiezoelectric crystal 130 (i.e. on the side facing away from theresonator 134). Specifically, a third adhesion layer 169 and a metallayer 170 are positioned on the back side of the piezoelectric crystal130. The layers 169 and 170 cover substantially all of the surface areaof the back side of the crystal 130. In the preferred embodiment, thethird adhesion layer 169 comprises an approximately 5000 Å thick layerof an alloy comprised of chrome and a nickel copper alloy, such as thealloys marketed under the trademarks Nickel 400™ or MONEL™. However,other materials and/or thicknesses could also be used as the thirdadhesion layer 169. The function of the third adhesion layer 169 is topromote adhesion of the metal layer 170 to the crystal 130.

Preferably, the metal layer 170 comprises silver, although otherelectrically conductive metals such as nickel could also be used.Generally, the crystal 130 is obtained from commercial sources withoutthe layers 169 and 170. The layers 169 and 170 are then applied to thepiezoelectric crystal 130 using a sputtering technique such as physicalvapor deposition (PVD). The layer 170 functions as an electrode toconduct electricity from a set of the spring/button connectors shown inFIG. 1, so as to set up a voltage across the crystal 130. Since thethird adhesion layer 169 is also electrically conductive, both of thelayers 169 and 170 actually function as an electrode.

In the preferred embodiment, the bonding layer 138 comprises pure indium(99.99%) such as is commercially available from Arconium or Indalloy.However, indium alloys containing varying amounts of impurity metals canalso be used, albeit with less satisfactory results. The benefit ofusing indium and its alloys is that indium possesses excellent shearproperties that allow dissimilar materials with different coefficientsof expansion to be attached together and experience thermal cycling(i.e. expansion and contraction at different rates) without damage tothe attached materials or to the resonator 134. The higher the purity ofthe indium, the better the shear properties of the system 124 will be.If the components of the acoustic transducer system 124 have similarcoefficients of expansion, then less pure indium can be used becauseshear factors are less of a concern. Less pure indium (i.e. alloys ofindium) has a higher melting point then pure indium and thus may be ableto tolerate more heat.

Depending upon the requirements of a particular cleaning task, thecomposition of the resonator 134 is selected from a group of chemicallyinert materials. For example, inert materials that work well as theresonator 134 include sapphire, quartz, silicon carbide, silicon nitrideand ceramics. One purpose of the resonator 134 is to separate (isolate)the piezoelectric crystal 130 from the fluid used in the cleaningprocess, so that the fluid does not damage the crystal 130.Additionally, it is unacceptable for the resonator 134 to chemicallyreact with the cleaning fluid. Thus, the material used as the resonator134 is usually dictated, at least in part, by the nature of the cleaningfluid. Sapphire is a desirable material for the resonator 134 when theitems to be cleaned by the megasonic cleaning apparatus require partsper trillion purity. For example, semiconductor wafers require this typeof purity. A hydrogen fluoride (HF) based cleaning fluid might be usedin a cleaning process of this type for semiconductor wafers.

The resonator 134 must also be able to transmit the acoustic energygenerated by the crystal 130 into the fluid. Therefore, the acousticproperties of the resonator 134 are important. Generally, it isdesirable that the acoustic impedance of the resonator 134 be betweenthe acoustic impedance of the piezoelectric crystal 130 and the acousticimpedance of the cleaning fluid in the fluid chamber 190 (shown in FIG.6). Preferably, the closer the acoustic impedance of the resonator 134is the acoustic impedance of the cleaning fluid, the better.

In one preferred embodiment, the resonator 134 is a piece of syntheticsapphire (a single crystal substrate of Al₂O₃). Preferably, the sapphireis high grade having a designation of 99.999% (5 9s+purity). Whensynthetic sapphire is used as the resonator 134, the thickness “v”,illustrated in FIG. 5 is approximately six millimeters. It should benoted that other forms of sapphire could be used as the resonator 134,such as rubies or emeralds. However, for practical reasons such as costand purity, synthetic sapphire is preferred. Additionally, other valuesfor the thickness “v” can be used.

In the preferred embodiment, the thickness “v” of the resonator 134 is amultiple of one-half of the wavelength of the acoustic energy emitted bythe piezoelectric crystal 130, so as to minimize reflectance problems.For example, “v” is approximately six millimeters for sapphire andacoustic energy of approximately 925 KHz. The wavelength of acousticenergy in the resonator 134 is governed by the relationship shown inequation 1 below:

λ=v _(L)/2f  (1)

where,

v_(L)=the velocity of sound in the resonator 134 (in mm/msec),

f=the natural frequency of the piezoelectric crystal 130 (in MHz)

λ=the wavelength of acoustic energy in the resonator 134.

From equation 1, it follows that when the composition of the resonatorchanges or when the natural resonance frequency of the crystal 130changes, the ideal thickness of the resonator 134 will change.Therefore, in all of the examples discussed herein, a thickness “v”which is a multiple of one-half of the wavelength λ could be used.

In another preferred embodiment, the resonator 134 is a piece of quartz(SiO₂-synthetic fused quartz). Preferably, the quartz has a purity of99.999% (5 9s+purity). When quartz is used as the resonator 134, thethickness “v”, illustrated in FIG. 5 is approximately three to sixmillimeters.

In another preferred embodiment, the resonator 134 is a piece of siliconcarbide (SiC). Preferably, the silicon carbide has a purity of 99.999%(5 9s+purity, semiconductor grade). When silicon carbide is used as theresonator 134, the thickness “v”, illustrated in FIG. 5 is approximatelysix millimeters.

In another preferred embodiment, the resonator 134 is a piece of siliconnitride (SiN). Preferably, the silicon nitride has a purity of 99.999%(5 9s+purity, semiconductor grade). When silicon nitride is used as theresonator 134, the thickness “v”, illustrated in FIG. 5 is approximatelysix millimeters.

In another preferred embodiment, the resonator 134 is a piece of ceramicmaterial. In this application, the term ceramic means alumina (Al₂O₃)compounds such as the material supplied by the Coors Ceramics Companyunder the designation Coors AD-998. Preferably, the ceramic material hasa purity of at least 99.8% Al₂O₃. When ceramic material is used as theresonator 134, the thickness “v”, illustrated in FIG. 5 is approximatelysix millimeters.

The acoustic transducer system 124 illustrated in FIG. 5 is prepared bythe following method: Assuming that the resonator 134 is sapphire, thesurface of the sapphire that will be adjacent to the adhesion layer 162is cleaned by abrasive blasting or chemical or sputter etching. Theadhesion layer 162 is then deposited on the resonator 134 using aphysical vapor deposition (“PVD”) technique, such as argon sputtering.More specifically, the chrome and nickel copper alloy (e.g. Nickel 400™or MONEL™) that comprise the layer 162 are co-sputtered onto to theresonator 134 so that the layer 162 is comprised of approximately 50%chrome and 50% nickel copper alloy. The wetting (silver) layer 142 isthen deposited on the adhesion layer 162 using argon sputtering. Aplating technique could also be used in this step.

The piezoelectric crystal 130 is preferably purchased without anyelectrode layers deposited on its surfaces. The third adhesion layer 169is then deposited on the crystal 130 using a PVD technique, such asargon sputtering. More specifically, the chrome and nickel copper alloythat comprise the layer 169 are co-sputtered onto to the crystal 130 sothat the layer 169 is comprised of approximately 50% chrome and 50%nickel copper alloy (e.g. Nickel 400™ or MONEL™). The electrode (silver)layer 170 is then deposited on the adhesion layer 169 using argonsputtering. A plating technique could also be used in this step.

Similarly, the first adhesion layer 158 is deposited on the oppositeface of the crystal 130 from the third adhesion layer 169 using a PVDtechnique like argon sputtering. More specifically, the chrome andnickel copper alloy that comprise the layer 158 are co-sputtered onto tothe crystal 130 so that the layer 158 is comprised of approximately 50%chrome and 50% nickel copper alloy. The wetting (silver) layer 150 isthen deposited on the adhesion layer 158 using argon sputtering. Aplating technique could also be used in this step.

The resonator 134 and the piezoelectric crystal 130 are both heated toapproximately 200° C., preferably by placing the resonator 134 and thecrystal 130 on a heated surface such as a hot-plate. When both pieceshave reached a temperature of greater than 160° C., solid indium isrubbed on the surfaces of the resonator 134 and the crystal 130 whichare to be attached. Since pure indium melts at approximately 157° C.,the solid indium liquefies when it is applied to the hot surfaces,thereby wetting the surfaces with indium. It is sometimes advantageousto add more indium at this time by using the surface tension of theindium to form a “puddle” of molten indium.

The resonator 134 and the piezoelectric crystal 130 are then pressedtogether so that the surfaces coated with indium are in contact witheach other, thereby forming the transducer system 124. The newly formedtransducer system 124 is allowed to cool to room temperature so that theindium solidifies. Preferably, the bonding (indium) layer 138 has athickness “g” which is just sufficient to form a void free bond. In thepreferred embodiment, “g” is approximately one mil (0.001 inches). It isthought that the thickness “g” should be as small as possible in orderto maximize the acoustic transmission, so thicknesses less than one milmight be even more preferable. Thicknesses up to about 0.01 inches couldbe used, but the efficiency of acoustic transmission drops off when thethickness “g” is increased.

Preferably, the transducer system 124 is allowed to cool with thepiezoelectric crystal 130 on top of the resonator 134 and the force ofgravity holding the two pieces together. Alternatively, a weight can beplaced on top of the piezoelectric crystal 130 to aide in the bonding ofthe indium. Another alternative is to place the newly formed transducersystem 124 in a clamping fixture.

Once the transducer system 124 has cooled to room temperature, anyexcess indium that has seeped out from between the piezoelectric crystal130 and the resonator 134, is removed with a tool or other means.

FIG. 6 illustrates a megasonic cleaning system 180 that utilizes theacoustic transducer system 124 (or the acoustic transducer 14). Thecleaning solution is contained within a tank 184. In the preferredembodiment, the tank 184 is square-shaped and has four vertical sides188. The resonator 134 forms part of the bottom surface of the tank 184.Other shapes can be used for the tank 184, and in other embodiments, theresonator 134 can form only aportion of the bottom surface of the tank184.

A fluid chamber 190 is the open region circumscribed by the sides 188.Since the sides 188 do not cover the top or bottom surfaces of the tank184, the sides 188 are said to partially surround the fluid chamber 190.The fluid chamber 190 holds the cleaning solution so the walls 188 andthe resonator 134 must make a fluid tight fit to prevent leakage. Theresonator 134 has an interface surface 191 which abuts the fluid chamber190 so that the interface surface 134 is in contact with at least someof the cleaning solution when cleaning solution is present in the fluidchamber 190. Obviously, the interface surface 191 is only in contactwith the cleaning solution directly adjacent to the surface 191 at anypoint in time.

In the preferred embodiment shown in FIG. 6, four piezoelectric crystals130 are used. In a typical preferred embodiment, each of the crystals isa rectangle having dimensions of 1 inch (width)×6 inch (length “k” inFIG. 5)×0.10 inch (thickness “s” in FIG. 5). Since the natural frequencyof the crystal changes with thickness, reducing the thickness will causethe natural frequency of the crystal to be higher. As was indicatedpreviously, other numbers of crystals can be used, other shapes for thecrystals can be used and the crystals can have other dimensions, such as1.25×7×0.10 inches or 1.5×8×0.10 inches. Each of the crystals 130 areattached to the resonator 134 by the plurality of layers describedpreviously with respect to FIG. 5. A gap 192 exists between eachadjacent crystal 130 to prevent coupling of the crystals.

The power for driving the crystals 130 is provided by a radiofrequency(RF) generator 194 (shown in FIG. 7). The electrical connections betweenthe RF generator 194 and the crystals 130 are provided by the pluralityof first spring/button connectors 38 and the plurality of secondspring/button connectors 42, as was explained previously with respect toFIGS. 1 and 3. The plurality of second spring/button connectors 42provide the positive (+) connection to the RF generator 194 and theplurality of first spring/button connectors 38 provide the negative (−)connection to the RF generator 194.

The transducer system 124 includes the step-region 195 (shown in FIG. 5)which is exactly analogous to the step-region 110 described previouslywith respect to FIG. 2. The step region 195 is a region on the secondwetting layer 142 that can be contacted by the upper buttons 90 of thespring/button connectors 38. Since all of the layers between the secondwetting layer 142 and the crystal 130 are electrically conductive (i.e.the layers 138, 150 and 158), contact with the step region 195 isequivalent to contact with the surface front surface of the crystal 130.The upper buttons 98 of the spring/button connectors 42 make electricalcontact with the metal layer 170 to complete the circuit for driving thePZT crystal 130. This circuit is represented schematically in FIG. 7.

Referring to FIG. 6, the printed circuit board (PCB) 34 and thepiezoelectric crystal 130 are positioned in a cavity 46 and aresurrounded by the housing 22 as was described previously with respect toFIG. 1. A plurality of items 196 to be cleaned are inserted through thetop of the tank 184.

The acoustic transducer system 124 (illustrated in FIG. 5) functions asdescribed below. It should be noted that the transducer 14 (illustratedin FIG. 2) works in the same manner as the acoustic transducer system124. However, for the sake of brevity, the components of the system 124are referenced in this discussion.

A radiofrequency (RF) voltage supplied by the RF generator 194 creates apotential difference across the piezoelectric crystal 130. Since this isan AC voltage, the crystal 130 expands and contracts at the frequency ofthe RF voltage and emits acoustic energy at this frequency. Preferably,the RF voltage applied to the crystal 130 has a frequency in the rangeof approximately 925 MHZ. However, RF voltages in the frequency range ofapproximately 0.4 to 2.0 MHZ can be used with the system 124, dependingon the thickness and natural frequency of the crystal 130. A 1000 wattRF generator such as is commercially available from Dressler Industriesof Strohlberg, Germany is suitable as the RF generator 194.

In the preferred embodiment, only one of the crystals 130 is driven bythe RF generator at a given time. This is because each of the crystals130 have different natural frequencies. In the preferred embodiment, thenatural frequency of each crystal 130 is determined and stored insoftware. The RF generator then drives the first crystal at the naturalfrequency indicated by the software for the first crystal. After aperiod of time (e.g. one millisecond), the RF generator 194 stopsdriving the first crystal and begins driving the second crystal at thenatural frequency indicated by the software for the second crystal 130.This process is repeated for each of the plurality of crystals.Alternatively, the natural frequencies for the various crystals 130 canbe approximately matched by adjusting the geometry of the crystals, andthen driving all of the crystals 130 simultaneously.

Most of the acoustic energy is transmitted through all of the layers ofthe system 124 disposed between the crystal 130 and the resonator 124,and is delivered into the cleaning fluid. However, some of the acousticenergy generated by the piezoelectric crystal 130 is reflected by someor all of these layers. This reflected energy can cause the layers toheat up, especially as the power to the crystal is increased.

In the present invention, the bonding layer 138 has an acousticimpedance that is higher than the acoustic impedance of other attachmentsubstances, such as epoxy. This reduces the amount of reflected acousticenergy between the resonator 134 and the bonding layer 138. This createstwo advantages in the present invention. First, less heat is generatedin the transducer system, thereby allowing more RF power to be appliedto the piezoelectric crystal 130. For example, in the transducer systemillustrated in FIG. 5, 25 to 30 watts/cm² can be applied to the crystal130 (for an individually excited crystal) without external cooling.Additionally, the system 124 can be run in a continuous mode withoutcooling (e.g. 30 minutes to 24 hours or more), thereby allowing bettercleaning to be achieved. In contrast, prior art systems useapproximately 7 to 8 watts/cm², without external cooling. Prior artmegasonic cleaning systems that operate at powers higher than 7 to 8watts/cm² in a continuous mode require external cooling of thetransducer.

Second, in the present invention, the reduced reflectance allows morepower to be delivered into the fluid, thereby reducing the amount oftime required in a cleaning cycle. For example, in the prior art, acleaning cycle for sub 0.5 micron particles generally requires fifteenminutes of cleaning time. With the present invention, this time isreduced to less than one minute for many applications. In general, theuse of the bonding (indium) layer 138 permits at least 90 to 98% of theacoustic energy generated by the piezoelectric crystal 130 to betransmitted into the cleaning fluid when the total power inputted to thepiezoelectric crystal 130 is in the range of 400 to 1000 watts (e.g. 50watts/cm² for a crystal 130 having an area of 20 cm²). In the preferredembodiment, the bonding (indium) layer 138 attenuates the acousticenergy that is transmitted into the volume of cleaning fluid by no morethan approximately 0.5 dB. It is believed that the system 124 can beused with power as high as 5000 watts. In general, the application ofhigher power levels to the piezoelectric crystal 130 results in fastercleaning times. It may also lead to more thorough cleaning.

Table 1 below indicates the power levels that can be utilized when theindicated materials are used as the resonator 134 in the system 124. Theinput wattage (effective power) is defined as the forward power into thecrystal 130 minus the reflected power back into the RF generator 194. Asindicated above, the system 124 allows at least approximately 90 to 98%of the input wattage to be transmitted into the cleaning solution.

TABLE 1 Resonator Input Wattage/cm² Quartz 12.5 watts/cm² Siliconcarbide or silicon nitride 20 watts/cm² Stainless steel 25 watts/cm²Ceramic 40 watts/cm² Sapphire 50 watts/cm²

Although the present invention has been described in terms of thepresently preferred embodiment, it is to be understood that suchdisclosure is not to be interpreted as limiting. Various alterations andmodifications will no doubt become apparent to those skilled in the artafter having read the above disclosure. Accordingly, it is intended thatthe appended claims be interpreted as covering all alterations andmodifications as fall within the true spirit and scope of the invention.

We claim:
 1. A megasonic cleaning system comprising: a container havinga fluid chamber for holding a volume of cleaning solution; a resonatorselected from the group consisting of sapphire, quartz, silicon carbide,silicon nitride and ceramic, the resonator having an interface surfacewhich abuts the fluid chamber so that the interface surface is incontact with at least some of the volume of cleaning solution when thevolume of cleaning solution is present in the fluid chamber; apiezoelectric crystal for generating acoustic energy in the frequencyrange of 0.4 to 2.0 MHz when power is applied to the piezoelectriccrystal; and a bonding layer comprised of indium positioned between theresonator and the piezoelectric crystal for attaching the piezoelectriccrystal to the energy transmitting member.
 2. The megasonic cleaningsystem of claim 1 wherein the indium in the bonding layer comprises atleast 99.99% pure indium.
 3. The megasonic cleaning system of claim 1wherein the resonator comprises sapphire.
 4. The megasonic cleaningsystem of claim 1 wherein the piezoelectric crystal comprises leadzirconate titanate.
 5. A megasonic transducer comprising: apiezoelectric crystal for generating acoustic energy in the frequencyrange of 0.4 to 2.0 MHz; a resonator adapted for positioning between thepiezoelectric crystal and a volume of cleaning fluid, the resonatorbeing selected from the group consisting of sapphire, quartz, siliconcarbide, silicon nitride and ceramic; and a bonding layer comprised ofindium positioned between the energy transmitting member and thepiezoelectric crystal for attaching the piezoelectric crystal to theresonator.
 6. The megasonic transducer of claim 5 wherein the indium inthe bonding layer comprises at least 99.99% pure indium.
 7. Themegasonic transducer of claim 5 wherein the resonator comprisessapphire.
 8. The megasonic transducer of claim 5 wherein thepiezoelectric crystal comprises lead zirconate titanate.
 9. A megasoniccleaning system comprising: a container having a fluid chamber forholding a volume of cleaning solution; a resonator selected from thegroup consisting of sapphire, quartz, silicon carbide, silicon nitrideand ceramic, the resonator having an interface surface which abuts thefluid chamber so that the interface surface is in contact with at leastsome of the volume of cleaning solution when the volume of cleaningsolution is present in the fluid chamber; a piezoelectric crystal forgenerating acoustic energy in the frequency range of 0.4 to 2.0 MHz whenpower is applied to the piezoelectric crystal; a bonding layer comprisedof indium positioned between the resonator and the piezoelectric crystalfor attaching the piezoelectric crystal to the energy transmittingmember, a first adhesion layer positioned in contact with a surface ofthe piezoelectric crystal; and a first wetting layer positioned betweenthe first adhesion layer and the bonding layer for helping the bondinglayer bond to the first adhesion layer.
 10. The megasonic cleaningsystem of claim 9 wherein the indium in the bonding layer comprises atleast 99.99% pure indium.
 11. The megasonic cleaning system of claim 9wherein the resonator comprises sapphire.
 12. The megasonic cleaningsystem of claim 9 wherein the piezoelectric crystal comprises leadzirconate titanate.
 13. The megasonic cleaning system of claim 9 whereinthe first adhesion layer comprises chromium.
 14. The megasonic cleaningsystem of claim 9 wherein the first wetting layer comprises silver. 15.The megasonic cleaning system of claim 9 further comprising: a secondadhesion layer positioned in contact with a surface of the resonator;and a second wetting layer positioned between the second adhesion layerand the bonding layer for helping the bonding layer bond to the secondadhesion layer.
 16. The megasonic cleaning system of claim 15 whereinthe second adhesion layer comprises chromium.
 17. The megasonic cleaningsystem of claim 15 wherein the second wetting layer comprises silver.18. The megasonic cleaning system of claim 15 further comprising: athird adhesion layer positioned in contact with a surface of thepiezoelectric crystal; and a metal layer positioned in contact with asurface of the third adhesion layer that faces away from thepiezoelectric crystal.
 19. The megasonic cleaning system of claim 18wherein the third adhesion layer comprises chromium.
 20. The megasoniccleaning system of claim 18 wherein the metal layer comprises silver.